In a basic form of an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot combustion gas is passed through a turbine mounted on the same shaft. The turbine includes a turbine disk (sometimes termed the “rotor”), upon which turbine blades are mounded. The flow of combustion gas turns the turbine by impingement against an airfoil section of the turbine blades, which turns the shaft and provides power to the compressor. Seals prevent the leakage of hot combustion gas around the turbine. After passing through the turbine, the hot combustion gas flows from the back of the engine, driving it and the aircraft forward.
In prior generations of aircraft gas turbine engines, the turbine disks and seal components operated at a sufficiently low temperature that hot corrosion was not a major concern. In current and advanced gas turbine engines, however, some of the components, such as the turbine disk and some of the seal components, are operated at a sufficiently high temperature that they are subjected to hot corrosion during operation. The corrodant is introduced into the turbine section of the engine in the hot combustion gases. The corrodant typically includes alkaline sulfate deposits that may have carbon as well.
Nickel-base superalloys are used as the materials of construction of some types of turbine disks and seal components. In service, the nickel-base superalloys are exposed to hot corrosion in the intermediate temperature range of about 1000° F. to about 1500° F. The compositions of the nickel-base superalloys are selected to achieve the required mechanical properties in service. However, the superalloys that have the desired mechanical properties are not sufficiently resistant to hot-corrosion damage. The hot-corrosion damage, if it becomes sufficiently severe, may cause the superalloy component to fail prematurely.
Environmentally resistant coatings are known for use with nickel-base superalloys operated at higher temperatures. Aluminum-containing diffusional and overlay coatings that oxidize to produce a protective aluminum oxide scale are widely used. However, these coatings are typically not suitable for use on wrought gas turbine components operated in the temperature range of about 1000° F. to about 1500° F., because they require higher deposition temperatures that adversely affect the mechanical properties of the heat-treated wrought nickel-base superalloys.
There is a need for an improved approach to the protection of nickel-base superalloys and other materials operated in a corrosive environment in the temperature range of about 1000° F. to about 1500° F. The new approach must be compatible with the processing of the component. The present invention fulfills this need, and further provides related advantages.